Rule to coordinate the pickup current of instantaneous overcurrent functions of numerical relays with downstream current-limiting fuses

Abstract

This article shows an analysis of the effect of let-thru currents of current-limiting fuses on numerical overcurrent relays. The fuse total clearing time is extremely fast in the current-limiting zone and, consequently, only the setting of instantaneous overcurrent function is the main concern. The peak value of fuse let-thru currents can be obtained from manufacturer data, the waveform of currents is approximated to a triangle, and the phenomenon duration is approximated to half-cycle. Under these conditions, it is numerically shown that the use of two well-known relay algorithms to compute the fundamental component magnitudes gives an equivalent current equal to the peak value divided by 3.5. This result is also demonstrated analytically. On the other hand, three commercial relays from different manufacturers were experimentally tested, with currents with these waveshapes, and these results also confirm the suitability of that numerical factor (i.e., 3.5) to obtain the equivalent current. Therefore, the use of this factor, in conjunction with a safety margin for the coordination, allows the selection of a suitable pick-up current for the instantaneous overcurrent function. Finally, it is also analytically shown that the correspondent numerical factor is 2.45 for relays whose overcurrent algorithm is based on true RMS values.